1. Field of Invention
This invention relates generally to methods and apparatus for averting corrosion of pipelines, and more specifically, the present invention relates to optimizing the detection and location of defects in coatings on the pipe structures without the necessity of excavation or local physical contact with the pipe.
2. Description of the Prior Art
Pipelines that are used to transport fluids, such as petroleum or other types of fluids or gases are often buried beneath the ground to preserve the above-ground real estate for other uses, as well as to protect the pipelines from the environment. The piping used to form the pipelines is coated to prevent corrosion. In fact, the coating integrity of the buried pipes is crucial to the prevention of outside surface (i.e., outside diameter (OD)) corrosion.
A disbonded coating defeats the security provided by cathodic protection on the pipe. The cathodic protection currents can no longer flow out through the coating to the cover soil as intended. Disbonds that are not repaired can lead to moisture ingression between the coating and the outer surface of the pipe, which can eventually result in corrosion and/or stress-corrosion cracking of the pipe. For a detailed understanding the effects of disbonds in pipeline coatings the reader is directed to the article Crude Oil Pipeline Rupture, Pipeline Investigation Report P99H0021, Transportation Safety Board of Canada, March 2002, the content of which is incorporated by reference herein in its entirety.
Corroded surfaces and stress-corrosion cracking along the pipe are much more costly to repair than simply repairing an area of the pipe having a coating that is disbonded. As a result, early detection of pipeline coating disbonds is necessary to maintain the integrity of a pipeline.
The detection and characterization of disbonded and/or defective coating using EIS (Electrochemical Impedance Spectroscopy) is well known. For example, the article entitled “Evaluation of Organic Coatings with Electrochemical Impedance Spectroscopy” by Loveday, et al., JCT Coatings Tech, October 2004, pp. 88-93 describes the general application of EIS to coatings. Moreover, an article entitled “Electrochemical Impedance of Coated Metal Undergoing Loss of Adhesion”, by Kendig, Martin W., et al, Electrochemical Impedance: Analysis and Interpretation, ASTM STP 1188, Scully, Silverman, and Kendig, eds., American Society for Testing and Materials, 1993, pp. 407-427 describes EIS responses to various coating conditions, including normal coating, coating at the onset of corrosion, and disbonded coating. The contents of both of these articles are incorporated by reference herein in their entirety.
The basic procedure is to measure the complex electrical impedance through the metal-to-coating interface at multiple frequencies followed by analysis of the impedance data. Displaying the data on Nyquist and Bode plots can reveal substantial information about the properties of the coating. Commercial software is available for fitting Nyquist-plot data to operator-selected equivalent circuits of the coating interface. The values of the resulting circuit components can reveal direct information on coating properties.
Application of EIS to pipeline coating inspection has been reported in an article entitled “The Study of Detection Technology and Instrument of Buried Pipeline Coating Defects”, by Shijiu, et al., Proceedings of the 4th World Congress on Intelligent Control and Automation, Institute of Electrical and Electronic Engineers, 2001, pp. 794-98, the content of which is incorporated by reference herein in its entirety. This article describes the ability to determine coating quality and type of defect using the measured EIS spectrum, as well as to differentiate between coating defects and coating disbonds using the EIS data.
EIS requires direct contact with the coating surface, necessitating excavation of the pipes, which can be burdensome and costly to perform. In an article by Murphy, J. C., et al., entitled “Magnetic Field Measurement of Corrosion Processes”, Journal of the Electrochemical Society, Vol. 135, No. 2, February 1988, pp. 310-313, it is disclosed that this problem of having to first excavate the pipes has been circumvented by the development of MEIS (Magnetically-detected Electrochemical Impedance Spectroscopy), the content of which is incorporated by reference herein in its entirety.
MEIS uses above-ground magnetometers to measure on-pipe current resulting from applying an AC voltage between the pipe and a remote ground-return electrode. A reference electrode is placed on the soil adjacent to the pipe. The actual pipe-to-soil voltage can be measured via this electrode independently of the effects of earthing resistance of the ground-return electrode.
The pipe-to-soil impedance of a segment of pipe can be determined by measuring the on-pipe current via a magnetometer sequentially positioned at two locations defining the ends of the segment, followed by calculating the differential net AC impedance of the segment. The pipe-to-reference electrode voltage is utilized along with the on-pipe current for these calculations. This procedure is described in the above-identified Murphy article which discloses: a) MEIS-measured Bode and Nyquist plots for each end of a pipe segment; and b) the resultant Bode and Nyquist plots for the segment itself. This procedure is also described in an article by Srinivasan, R. et al., entitled “Corrosion Detection on Underground Gas Pipeline by Magnetically Assisted AC Impedance”, Materials Performance, vol. 30, no. 3, NACE, Houston, Tex., 1991, pp. 14-18, the contents of which are incorporated by reference herein in their entirety.
Standard EIS analysis techniques can be then be applied to the pipe-segment's impedance. The equivalent circuit of the segment's pipe-to-soil interface can be determined via conventional analysis of Bode and Nyquist plots of this impedance data. This analysis can utilize a Randles equivalent circuit or other equivalent circuit of the coating interface. The component values of the equivalent circuit can be analyzed to determine integrity of the coating, including degree of disbond or damage, as reported in the above mentioned articles by Kendig, et al. and Shijiu, et al. For additional information describing the use of MEIS technology for determining corrosion rate measurements, the reader is directed to U.S. Pat. No. 5,126,654 to Murphy et al., the content of which is also incorporated by reference herein in its entirety. The Murphy patent describes the use of MEIS to calculate the resistance and capacitance of the pipe-to-soil interface, and using these values to characterize the corrosion rate.